Anhydride-cured epoxides are characterized by good overall physical, electrical and chemical properties as well as by high heat-distortion temperatures which in many cases exceed the best values of aromatic diamine-epoxy systems. Another decided advantage is that, unlike amine-curatives, the anhydrides are not skin sensitizing compounds. The main disadvantage of anhydride-epoxy systems is their long cure cycle at elevated temperatures with the subsequent risk of vaporization and loss of curing agent. While active chromium III tricarboxylate salts having unoccupied coordination sites have been found to greatly accelerate the curing of organic acid-epoxy compound reactions, (These catalysts are disclosed and claimed in another copending application, Ser. No. 389,079 filed Aug. 17, 1973 and, per se, do not form a part of the present invention) these catalysts alone are not suitable catalysts for the reaction of anhydrides with oxiranes. With such catalysts, relatively long cure cycles at higher temperatures are required and the cured resins are of poor quality.
The present invention provides a new group of synergistic catalyst mixtures for promoting rapid reactions of oxirane compounds with crosslinking agents and chain extension agents. According to the present invention oxirane curing can take place at relatively low temperatures, such as below 100.degree.C and even as low as room temperature.